Vacuum-jacketed cryogenic storage vessels are used to store liquified gasses at extremely low temperatures. To maintain the gas in a liquid state, heat transfer from the ambient environment to the liquified gas must be restricted. To accomplish this, the cryogenic vessels comprise an inner vessel within an outer vessel, the walls of the vessels being separated by an insulating vacuum and radiation barrier. A neck tube connects the inner vessel to the outer vessel to permit filling and extraction of liquified gas from storage in the inner vessel. Most prior art cryogenic vessels include a straight, thin wall tube between the inner and outer vessels.
The conduction of heat to the inner vessel affects the natural evaporation rate (NER) of the cryogenic vessel. The more heat that is conducted from the outside to the inner vessel, the faster the rate of boil-off of the cryogenic liquid or the greater the NER. Unfortunately, the neck tube provides a thermal conductive path from the ambient environment to the inner vessel.
The amount of heat transfer through the neck tube depends on a number of factors. First, heat transfer is dependent on the thermal conductivity of the neck tube material. In addition, heat transfer is proportional to the mass of the neck tube cross-section and inversely proportional to the length of the neck tube. Some prior art cryogenic vessels have incorporated bellows neck tube assemblies to increase the length of the tubing, and thus the thermal transfer path, but not the distance between the inner and outer vessel.